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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

ITER Toroidal Interferometer and Polarimeter (TIP) beam refraction in 3D density profiles

Calculations of the expected refraction of the five ITER Toroidal Interferometer and Polarimeter (TIP) chords are presented for a range of conditions including high-density axisymmetric, ELMing H-mode cases and shattered pellet disruption mitigation cases. The calculations are carried out with a newly developed ray tracing code capable of following TIP’s 10.59 μm laser beams through arbitrary 3D electron density profiles. Using JOREK simulations of ELM density perturbations in a 15MA ITER baseline plasma, it is shown that refraction from ELMs is expected to be negligible. It is found, however, that TIP interferometers will be able to clearly resolve the line-integrated density perturbation from ELMs and contribute to the ITER measurement requirement “14. H-mode, ELMs and L-H mode transition indicator”. Calculations of the expected refraction in a NIMROD simulated shattered pellet disruption mitigation scenario with peak local electron densities of n e = 3.7 x 10 21 m -3 (max line-averaged densities of n avg = 1.3 x 10 21 m -3 ) also show tolerable refraction and it is likely that most, if not all chords, would avoid signal loss. In conclusion, for both axisymmetric and structured plasmas with line-averaged densities in the mid to upper 10 21 m -3 range, values likely present only during disruption mitigation, refraction becomes significant and could limit the ability of TIP to make reliable density measurements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

DISCRIMINATING BETWEEN CLOUDY, HAZY, AND CLEAR SKY EXOPLANETS USING REFRACTION

We propose a method to distinguish between cloudy, hazy, and clear sky (free of clouds and hazes) exoplanet atmospheres that could be applicable to upcoming large aperture space- and ground-based telescopes such as the James Webb Space Telescope (JWST) and the European Extremely Large Telescope (E-ELT). These facilities will be powerful tools for characterizing transiting exoplanets, but only after a considerable amount of telescope time is devoted to a single planet. A technique that could provide a relatively rapid means of identifying haze-free targets (which may be more valuable targets for characterization) could potentially increase the science return for these telescopes. Our proposed method utilizes broadband observations of refracted light in the out-of-transit spectrum. Light refracted through an exoplanet atmosphere can lead to an increase of flux prior to ingress and subsequent to egress. Because this light is transmitted at pressures greater than those for typical cloud and haze layers, the detection of refracted light could indicate a cloud- or haze-free atmosphere. A detection of refracted light could be accomplished in <10 hr for Jovian exoplanets with JWST and <5 hr for super-Earths/mini-Neptunes with E-ELT. We find that this technique is most effective for planets with equilibrium temperatures between 200 and 500 K, which may include potentially habitable planets. A detection of refracted light for a potentially habitable planet would strongly suggest the planet was free of a global cloud or haze layer, and therefore a promising candidate for follow-up observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Refractive index of lithium fluoride at high dynamic stresses

Alkali halides are prototypical ionic solids whose refractive index is a fundamental property related to their lattice structure and ionic polarizability. In particular, lithium fluoride (LiF) has the largest band gap of any known transparent material and maintains transparency at high pressures, making it well suited for use as an optical window in dynamic compression experiments. While empirical fits to the measured density dependence of the refractive index have been provided, a model that is valid over the entire pressure range of experiments and based on a polarization-based theoretical description is lacking. We present a refractive index model for dynamically compressed LiF based on the Lorentz-Lorenz equation, where the molecular polarizability is determined using a single oscillator model with a strain polarizability parameter Λ=0.73. Here, we show that our modeling approach provides an excellent match to the LiF refractive index data for both shock and ramp compression experiments to 900 GPa (density compression greater than threefold). Additionally, updated fits are provided to determine the refractive index correction for LiF windows used in dynamic compression experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Sci—Fri PM: Dosimetry—01: Radiation-induced refraction artefacts in the optical CT readout of polymer gel dosimeters

Polymer gel dosimeters (PGDs) are a desirable tool for the verification of advanced radiotherapy treatments. Fully 3D, deformable, and tissue-equivalent, the PGD polymerizes wherever it absorbs dose. To measure the dose absorbed by a PGD, optical computed tomography (CT) can be used to evaluate, in full 3D, the opacity distribution that coincides with polymerization. In addition to an increase in opacity with dose, an increase in refractive index (RI) is also known to occur in irradiated polymer gels. The increase in RI is slight and was previously assumed insignificant. This work reveals the effects that radiation-induced RI changes can have on the optical CT readout of PGDs. A fan-beam optical CT scanner was used to image a cylindrical PGD irradiated by a pair of 3×3 cm{sup 2}, 6 MV photon beams in an orthogonal arrangement. Investigative scans were performed to evaluate refraction errors occurring: i) within the plane, and ii) out of the plane of the fan-beam. In-plane refraction was shown to cause distinct streaking artefacts along dose gradients (i.e. RI gradients) due to higher intensity rays being refracted into more opaque regions. Out-of-plane refraction was shown to produce severe, widespread artefacts due to rays missing the detector array. An iterative Savitzky-Golay filtering technique was developed to reduce both types of artefacts by specifically targeting structured errors in sinogram space. Results introduce a new category of imaging artefacts to be aware of when using optical CT for PGD readout.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Further Studies on the Effect of SiN x Refractive Index and Emitter Sheet Resistance on Potential-Induced Degradation

We present the impacts of silicon nitride (SiNx) antireflection coating refractive index and emitter sheet resistance on potential-induced degradation of the shunting type (PID-s). Previously, it has been shown that the cell becomes more PID-s-susceptible as the refractive index decreases or the emitter sheet resistance increases. To verify the effect of refractive index on PID-s, we fabricated cells with varying SiN x refractive index (1.87, 1.94, 2.05) on typical p-type base solar cells with ~60 Ω/sq emitters. However, none of these cells showed output power degradation, regardless of the refractive index. Further investigation of the emitter showed that the PID-s was suppressed at ~60 Ω/sq due to the extremely high surface phosphorus concentration (6 x 10 21 cm -3 ), as measured by secondary ion mass spectrometry. Furthermore, PID-s was observed on cells possessing a high emitter sheet resistance (~80 Ω/sq). In conclusion, the emitter surface phosphorus concentration plays an important role in determining PID-s susceptibility.

14 SOLAR ENERGY↗
NASA NTRS1973

Refraction effects of atmosphere on geodetic measurements to celestial bodies

The problem is considered of obtaining accurate values of refraction corrections for geodetic measurements of celestial bodies. The basic principles of optics governing the phenomenon of refraction are defined, and differential equations are derived for the refraction corrections. The corrections fall into two main categories: (1) refraction effects due to change in the direction of propagation, and (2) refraction effects mainly due to change in the velocity of propagation. The various assumptions made by earlier investigators are reviewed along with the basic principles of improved models designed by investigators of the twentieth century. The accuracy problem for various quantities is discussed, and the conclusions and recommendations are summarized.

Joshi, C. S.↗
NASA NTRS1976

Comparison of remotely sensed continental-shelf wave spectra with spectra computed by using a wave refraction computer model

An initial attempt was made to verify the Langley Research Center and Virginia Institute of Marine Science mid-Atlantic continental-shelf wave refraction model. The model was used to simulate refraction occurring during a continental-shelf remote sensing experiment conducted on August 17, 1973. Simulated wave spectra compared favorably, in a qualitative sense, with the experimental spectra. However, it was observed that most of the wave energy resided at frequencies higher than those for which refraction and shoaling effects were predicted, In addition, variations among the experimental spectra were so small that they were not considered statistically significant. In order to verify the refraction model, simulation must be performed in conjunction with a set of significantly varying spectra in which a considerable portion of the total energy resides at frequencies for which refraction and shoaling effects are likely.

Poole, L. R.↗
NASA NTRS1980

Determination of the complex refractive index and size distribution of atmospheric particulates from bistatic-monostatic lidar and solar radiometer measurements

A method is presented for inferring both the size distribution and the complex refractive index of atmospheric particulates from combined bistatic-monostatic lidar and solar radiometer observations. The basic input measurements are spectral optical depths at several visible and near-infrared wavelengths as obtained with a solar radiometer and backscatter and angular scatter coefficients as obtained from a biostatic-monostatic lidar. The spectral optical depth measurements obtained from the radiometer are mathematically inverted to infer a columnar particulate size distribution. Advantage is taken of the fact that the shape of the size distribution obtained by inverting the particulate optical depth is relatively insensitive to the particle refractive index assumed in the inversion. Bistatic-monostatic angular scatter and backscatter lidar data are then processed to extract an optimum value for the particle refractive index subject to the constraint that the shape of the particulate size distribution be the same as that inferred from the solar radiometer data. Specifically, the scattering parameters obtained from the bistatic-monostatic lidar data are compared with corresponding theoretical computations made for various assumed refractive index values. That value which yields best agreement, in a weighted least squares sense, is selected as the optimal refractive index estimate. The results of this procedure applied to a set of simulated measurements as well as to measurements collected on two separate days are presented and discussed.

Reagan, J. A.↗
NASA NTRS1984

Spatial variation of stratospheric aerosol acidity and model refractive index - Implications of recent results

Recent experimental results indicate that little or no solid ammonium sulfate is present in background stratospheric aerosols. Other results allow straightforward calculation of sulfuric acid/water droplet properties (acidity, specific gravity, refractive index) as functions of stratospheric temperature and humidity. These results are combined with a variety of latitudinal and seasonal temperature and humidity profiles to obtain corresponding profiles of droplet properties. These profiles are used to update a previous model of stratospheric aerosol refractive index. The new model retains the simplifying approximation of vertically constant refractive index in the inner stratosphere, but has sulfuric acid/water refractive index values that significantly exceed the previously used room temperature values. Mean conversion ratios (e.g., extinction-to-number, backscatter-to-volume) obtained using Mie scattering calculations with the new refractive indices are very similar to those obtained for the old indices, because the effects of deleting ammonium sulfate and increasing acid indices tend to cancel each other.

Russell, P. B.↗
NASA NTRS1990

Distorted-wave born approximation calculations for turbulence scattering in an upward-refracting atmosphere

Weiner and Keast observed that in an upward-refracting atmosphere, the relative sound pressure level versus range follows a characteristic 'step' function. The observed step function has recently been predicted qualitatively and quantitatively by including the effects of small-scale turbulence in a parabolic equation (PE) calculation. (Gilbert et al., J. Acoust. Soc. Am. 87, 2428-2437 (1990)). The PE results to single-scattering calculations based on the distorted-wave Born approximation (DWBA) are compared. The purpose is to obtain a better understanding of the physical mechanisms that produce the step-function. The PE calculations and DWBA calculations are compared to each other and to the data of Weiner and Keast for upwind propagation (strong upward refraction) and crosswind propagation (weak upward refraction) at frequencies of 424 Hz and 848 Hz. The DWBA calculations, which include only single scattering from turbulence, agree with the PE calculations and with the data in all cases except for upwind propagation at 848 Hz. Consequently, it appears that in all cases except one, the observed step function can be understood in terms of single scattering from an upward-refracted 'skywave' into the refractive shadow zone. For upwind propagation at 848 Hz, the DWBA calculation gives levels in the shadow zone that are much below both the PE and the data.

Gilbert, Kenneth E.↗
NASA NTRS1999

Optical Evaluation of a Refractive Secondary Concentrator

Refractive secondary concentrators are being considered for solar thermal applications because of their ability to archive maximum efficiency through the use of total internal reflection for the concentration and distribution of solar energy. A prototype refractive secondary concentrator was built based on ray tracing analysis to demonstrate this collection and distribution concept. The design included a conical secondary concentrator and a faceted extractor. The objective of this effort was to functionally evaluate the performance of the refractive secondary concentrator/extractor prototype and to compare the results with modeling. Most of the light was found to exit the refractive secondary concentrator through the extractor. In addition, the degree of attenuation encountered by the light as it passed through the refractive secondary concentrator was of interest. Quantifying optical output and validating the modeling will provide further understanding of the efficiency of the prototype and will provide insight for additional design and materials selection activities.

Jaworske, Donald A.↗
NASA NTRS1999

Transient Thermal Analysis of a Refractive Secondary Solar Concentrator

A secondary concentrator is an optical device that accepts solar energy from a primary concentrator and further intensifies and directs the solar flux. The refractive secondary is one such device; fabricated from an optically clear solid material that can efficiently transmit the solar energy by way of refraction and total internal reflection. When combined with a large state-of-the-art rigid or inflatable primary concentrator, the refractive secondary enables solar concentration ratios of 10,000 to 1. In support of potential space solar thermal power and propulsion applications, the NASA Glenn Research Center is developing a single-crystal refractive secondary concentrator for use at temperatures exceeding 2000K. Candidate optically clear single-crystal materials like sapphire and zirconia are being evaluated for this application. To support this evaluation, a three-dimensional transient thermal model of a refractive secondary concentrator in a typical solar thermal propulsion application was developed. This paper describes the model and presents thermal predictions for both sapphire and zirconia prototypes. These predictions are then used to establish parameters for analyzing and testing the materials for their ability to survive thermal shock and stress.

Geng, Steven M.↗
NASA NTRS1999

Refractive Secondary Concentrators for Solar Thermal Applications

The NASA Glenn Research Center is developing technologies that utilize solar energy for various space applications including electrical power conversion, thermal propulsion, and furnaces. Common to all of these applications is the need for highly efficient, solar concentration systems. An effort is underway to develop the innovative single crystal refractive secondary concentrator, which uses refraction and total internal reflection to efficiently concentrate and direct solar energy. The refractive secondary offers very high throughput efficiencies (greater than 90%), and when used in combination with advanced primary concentrators, enables very high concentration ratios (10,0(X) to 1) and very high temperatures (greater than 2000 K). Presented is an overview of the refractive secondary concentrator development effort at the NASA Glenn Research Center, including optical design and analysis techniques, thermal modeling capabilities, crystal materials characterization testing, optical coatings evaluation, and component testing. Also presented is a discussion of potential future activity and technical issues yet to be resolved. Much of the work performed to date has been in support of the NASA Marshall Space Flight Center's Solar Thermal Propulsion Program. The many benefits of a refractive secondary concentrator that enable efficient, high temperature thermal propulsion system designs, apply equally well to other solar applications including furnaces and power generation systems such as solar dynamics, concentrated thermal photovoltaics, and thermionics.

Wong, Wayne A.↗
NASA NTRS1957

Reflection and Refraction of Acoustic Waves by a Shock Wave

The presence of sound waves in one or the other of the fluid regions on either side of a shock wave is made apparent, in the region under superpressure, by acoustic waves (reflected or refracted according to whether the incident waves lie in the region of superpressure or of subpressure) and by thermal waves. The characteristics of these waves are calculated for a plane, progressive, and uniform incident wave. In the case of refraction, the refracted acoustic wave can, according to the incidence, be plane, progressive, and uniform or take the form of an 'accompanying wave' which remains attached to the front of the shock while sliding parallel to it. In all cases, geometrical constructions permit determination of the kinematic characteristics of the reflected or refractive acoustic waves. The dynamic relationships show that the amplitude of the reflected wave is always less than that of the incident wave. The amplitude of the refracted wave, whatever its type, may in certain cases be greater than that of the incident wave.

Brillouin, J.↗
NASA NTRS2004

CHARMS: The Cryogenic, High-Accuracy Refraction Measuring System

The success of numerous upcoming NASA infrared (IR) missions will rely critically on accurate knowledge of the IR refractive indices of their constituent optical components at design operating temperatures. To satisfy the demand for such data, we have built a Cryogenic, High-Accuracy Refraction Measuring System (CHARMS), which, for typical 1R materials. can measure the index of refraction accurate to (+ or -) 5 x 10sup -3 . This versatile, one-of-a-kind facility can also measure refractive index over a wide range of wavelengths, from 0.105 um in the far-ultraviolet to 6 um in the IR, and over a wide range of temperatures, from 10 K to 100 degrees C, all with comparable accuracies. We first summarize the technical challenges we faced and engineering solutions we developed during the construction of CHARMS. Next we present our "first light," index of refraction data for fused silica and compare our data to previously published results.

Frey, Bradley↗
NASA NTRS2002

Refractive Secondary Solar Concentrator Demonstrated High-Temperature Operation

Space applications that utilize solar thermal energy--such as electric power conversion systems, thermal propulsion systems, and furnaces--require highly efficient solar concentration systems. The NASA Glenn Research Center is developing the refractive secondary concentrator, which uses refraction and total internal reflection to efficiently concentrate and direct solar energy. When used in combination with advanced lightweight primary concentrators, such as inflatable thin films, the refractive secondary concentrator enables very high system concentration ratios and very high temperatures. Last year, Glenn successfully demonstrated a secondary concentrator throughput efficiency of 87 percent, with a projected efficiency of 93 percent using an antireflective coating. Building on this achievement, Glenn recently successfully demonstrated high-temperature operation of the secondary concentrator when it was used to heat a rhenium receiver to 2330 F. The high-temperature demonstration of the concentrator was conducted in Glenn's 68-ft long Tank 6 thermal vacuum facility equipped with a solar simulator. The facility has a rigid panel primary concentrator that was used to concentrate the light from the solar simulator onto the refractive secondary concentrator. NASA Marshall Space Flight Center provided a rhenium cavity, part of a solar thermal propulsion engine, to serve as the high-temperature receiver. The prototype refractive secondary concentrator, measuring 3.5 in. in diameter and 11.2 in. long, is made of single-crystal sapphire. A water-cooled splash shield absorbs spillage light outside of the 3.5-in. concentrator aperture. Multilayer foil insulation composed of tungsten, molybdenum, and niobium is used to minimize heat loss from the hightemperature receiver. A liquid-cooled canister calorimeter is used to measure the heat loss through the multilayer foil insulation.

Wong, Wayne A.↗
NASA NTRS2007

Cryogenic Temperature-dependent Refractive Index Measurements of N-BK7, BaLKN3, and SF15 for NOTES PDI

In order to enable high quality lens designs using N-BK7, BaLKN3, and SF15 at cryogenic temperatures, we have measured the absolute refractive index of prisms of these three materials using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center, as a function of both wavelength and temperature. For N-BK7, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 50 to 300 K at wavelengths from 0.45 to 2.7 micrometers; for BaLKN3 we cover temperatures ranging from 40 to 300 K and wavelengths from 0.4 to 2.6 micrometers; for SF15 we cover temperatures ranging from 50 to 300 K and wavelengths from 0.45 to 2.6 micrometers. We compare our measurements with others in the literature and provide temperature-dependent Sellmeier coefficients based on our data to allow accurate interpolation of index to other wavelengths and temperatures. While we generally find good agreement (plus or minus 2 x 10(exp -4) for N-BK7, less than 1 x 10(exp -4) for the other materials) at room temperature between our measured values and those provided by the vendor, there is some variation between the datasheets provided with the prisms we measured and the catalog values published by the vendor. This underlines the importance of measuring the absolute refractive index of the material when precise knowledge of the refractive index is required.

Frey, Bradley J.↗
NASA NTRS2006

Temperature-dependent Refractive Index of Silicon and Germanium

Silicon and germanium are perhaps the two most well-understood semiconductor materials in the context of solid state device technologies and more recently micromachining and nanotechnology. Meanwhile, these two materials are also important in the field of infrared lens design. Optical instruments designed for the wavelength range where these two materials are transmissive achieve best performance when cooled to cryogenic temperatures to enhance signal from the scene over instrument background radiation. In order to enable high quality lens designs using silicon and germanium at cryogenic temperatures, we have measured the absolute refractive index of multiple prisms of these two materials using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center, as a function of both wavelength and temperature. For silicon, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 20 to 300 K at wavelengths from 1.1 to 5.6 pin, while for germanium, we cover temperatures ranging from 20 to 300 K and wavelengths from 1.9 to 5.5 microns. We compare our measurements with others in the literature and provide temperature-dependent Sellmeier coefficients based on our data to allow accurate interpolation of index to other wavelengths and temperatures. Citing the wide variety of values for the refractive indices of these two materials found in the literature, we reiterate the importance of measuring the refractive index of a sample from the same batch of raw material from which final optical components are cut when absolute accuracy greater than k5 x 10" is desired.

Frey, Bradley J.↗